Fixed nested tab elements li activation

Added closest li instead of direct parent so that you can nest multiple elements within tabs, such as button groups
Dropdown menu in justified button-group

It seems that dropdown-menu in justified button-group appears at the far
left of the group. Check the problem in [Bootstrap
site](http://getbootstrap.com/components/#btn-groups-justified).
move Grunt info from README into HTML docs

Fixes #13470
Merge pull request #13531 from twbs/grunt-html-docs

move Grunt info from README into HTML docs
Update .travis.yml
Remove outdated rtl nav links
Merge pull request #13536 from Quy/master

Remove outdated rtl nav links
Fixed affix-bottom positioning

Set top position to (scrollHeight - this.$element.height() - offsetBottom).
grunt dist
Customizer: make utilities.less inclusion mandatory

Fixes #13305.
Merge pull request #13543 from twbs/fix-13305

Customizer: make utilities.less inclusion mandatory
add role="tablist" to .nav-tabs in examples to improve accessibility

Per https://github.com/paypal/bootstrap-accessibility-plugin#tab-panel
Credit: https://github.com/paypal/bootstrap-accessibility-plugin
add role="tab" to .nav-tabs [data-toggle="tab"]

Per https://github.com/paypal/bootstrap-accessibility-plugin#tab-panel
Credit: https://github.com/paypal/bootstrap-accessibility-plugin
add role="button" to [data-slide="prev"],[data-slide="next"] to improve accessibility

Per https://github.com/paypal/bootstrap-accessibility-plugin/blob/master/README.md#carousel
Credit: https://github.com/paypal/bootstrap-accessibility-plugin
Merge pull request #13559 from twbs/carousel-role

add role="button" to [data-slide="prev"],[data-slide="next"] to improve accessibility
Merge pull request #13558 from twbs/nav-tabs-role-tablist

add some ARIA roles to tabs for improved accessibility
Update html5shiv to 3.7.2.
Squashed commit of the following:

commit 81324491f4913d6797c3c9e4df83a1aefd7d9116
Author: Daniel <dano.husar@gmail.com>
Date:   Sat May 10 17:37:29 2014 +0100

    use opacity mixin

commit 694b0a3fcd196e8cba9e4c23850f903c0804cb44
Author: Daniel Husar <dano.husar@gmail.com>
Date:   Fri May 9 13:47:35 2014 +0100

    Update button-groups.less
Merge pull request #13541 from gpakosz/patch-2

Fixed affix-bottom positioning
Merge pull request #13549 from danielhusar/master

IE8 button group support